An experimental study of oil-water flow in horizontal and inclined pipes

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Lene Amundsen

An experimental study of oil-water flow in horizontal and inclined pipes

ISBN 978-82-471-2783-4 (printed ver.) ISBN 978-82-471-2784-1 (electronic ver.) ISSN 1503-8181

NTNU Norwegian University of Science and Technology Thesis for the degree of doctor philosophiae Faculty Engineering Science and Technology Department of Energy and Process Engineering

Doctoral theses at NTNU, 2011:123

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1 ABSTRACT

The large discrepancies between model predictions and experimental data reported in the literature reveal that the physics of oil-water flow is complex and not yet fully understood.

The knowledge of oil-water flow behaviour is essential in both field development and in production of gas and oil. Oil and water models, which are based on knowledge about the flow characteristics, are used to predict important parameters such as flow rates and pressure drop in production pipelines. Reliable simulation tools ensure safe production and high regularity. To find the features of the oil-water flow, a flow facility was built to enable visual observations and measurements of pressure gradient, local volume fractions, velocities and turbulence in horizontal and inclined pipes. The pipe inclination is an essential parameter since both production pipelines and wells can incline from 0° to 90° angle. Due to the uneven seabed the production pipelines contains sections with different inclinations, which normally varies between -10° to +10° angle. In the present work the effect of pipe inclination (-10° to +10°) and input water cut was studied at a constant mixture velocity equal to 1 m/s. A mixture velocity of 1 m/s is representative for the velocity in wells and production pipelines. A gamma densitometer measured the vertical phase distributions at input water cut from 10 to 90% and a Laser Doppler Anemometer measured the vertical velocity and turbulence distributions in the centre line of the pipe at 25 and 50% input water cut. The experiments were conducted in a 2-in. stainless steel pipe at atmospheric conditions and the fluids used were Exxsol D60 and tap water. An oil-water model was validated against the horizontal pipe flow data from this study at a mixture velocity equal to 1 m/s and against the horizontal pipe flow data of Elseth (2001) taken at higher mixture velocities equal to 2 and 3 m/s.

The experimental results give a good foundation for model validation and development. The pressure drop experiments showed that the frictional pressure gradient in two-phase flow was higher than that of single-phase oil flow, except in upward flow at low input water cuts. A steep frictional pressure gradient increase at high input water cut was observed in horizontal, downward and upward flow. The peak was observed at approximately 80% input water cut and was in the same order of magnitude; 1.7 times larger than that of pure oil. The abrupt increase is attributed to the increase in effective viscosity of the upper dispersed layer and to a non-radial phase distribution. This study presents a classification approach to determine the flow pattern. The method uses visual observations, the measured phase distributions and a

1 ABSTRACT

An experimental study of oil-water flow in horizontal and inclined pipes Abstract

ABSTRACT

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2

pre-defined phase inversion point to determine whether the flow is stratified or dispersed. The identified flow regimes in the parameter range studied were three different stratified flows with different degrees of dispersion and a plug flow. The most occurring flow regime in horizontal and inclined pipe flow was stratified flow with mixing at the interface (ST & MI).

An error propagation analysis of the measured phase fractions showed that the accuracy is best in the centre of the pipe and that it decreases when approaching the wall due to increased absorption in the wall compared with the liquid phase. The calculated slip ratio and LDA velocity profile measurements showed that the oil phase is the fastest moving phase in upward flow. The velocity and turbulence profiles were greatly influenced by large inclination angles (±5° and ±10°), while smaller inclinations (±1°) had less effect. The effect of pipe inclination on the velocity and turbulence profiles was larger at 50% input water cut compared to 25%. A dampening effect of the cross-moments occurred in both horizontal and downward moving flow at 25 and 50% input water cut.

The oil-water model presented is an important step in improving of existing simulations tools used in the oil and gas industry. The oil-water flow model is developed by Schulkes (2000) and compared with experimental phase fractions, pressure gradient and slip ratio in horizontal pipe flow. The model is a combined two-fluid model and dispersion model and calculates the vertical phase fraction distributions, slip ratios and pressure gradients as function of input water cut. The dispersion model is a balance between turbulent and buoyancy forces.

Experimental data published by Elseth (2001) were used in the comparison at higher flow velocities (2 and 3 m/s). In horizontal flow at the highest mixture velocity equal to 3 m/s the flow pattern predictions matched the experimental data perfectly. The predictions at lower velocities showed larger deviations. Although the model was able to predict important trends as increasing degree of mixing between oil and water with increasing mixture velocity, the model predictions of the frictional pressure gradient and phase fractions became gradually worse with increasing mixture velocity. However the slip ratio deviations do not increase with increasing mixture velocity as observed for the pressure gradient and phase fractions. The frictional pressure gradient predictions agreed reasonable well at 1 m/s except at high input water cuts. Here, the model was unable to reproduce the sharp increase in the frictional pressure gradient due to dispersed layer in top of the pipe. Strong correlations between the experimental and predicted phase fractions were also seen at 1 m/s, except at 50% input water cut and water cuts larger than 80%. The frictional pressure gradient was in general over

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3 predicted at 2 and 3 m/s. The deviations observed between predictions and experiments could be due to a wrong model assumption of a non-radial phase distribution (structured layer), inadequate closure relations or due to additional mechanisms such as flocculation, interfacial waves, surfactants or lift forces that are not included in the model. The main contribution to the deviations in frictional pressure drop at 1 m/s at high input water cuts is attributed to the increase in viscosity in the upper dispersed phase and the effect of drop entrainment and the oil-water interface on the closure relations for single-phase shear stresses. Possibly explanation for the observed differences in phase distribution with increasing velocity is the assumption of a dilute phase for calculation of the closure relations for the turbulent diffusion coefficient and buoyancy forces. The closure relations will in addition be affected by the existence of the oil-water interface. The poor correlations between pressure drop predictions and experiments at higher velocities is most likely caused by the affect of multiphase flow behaviour on the single-phase shear stress correlations.

predicted at 2 and 3 m/s. The deviations observed between predictions and experiments could be due to a wrong model assumption of a non-radial phase distribution (structured layer), inadequate closure relations or due to additional mechanisms such as flocculation, interfacial waves, surfactants or lift forces that are not included in the model. The main contribution to the deviations in frictional pressure drop at 1 m/s at high input water cuts is attributed to the increase in viscosity in the upper dispersed phase and the effect of drop entrainment and the oil-water interface on the closure relations for single-phase shear stresses. Possibly explanation for the observed differences in phase distribution with increasing velocity is the assumption of a dilute phase for calculation of the closure relations for the turbulent diffusion coefficient and buoyancy forces. The closure relations will in addition be affected by the existence of the oil-water interface. The poor correlations between pressure drop predictions and experiments at higher velocities is most likely caused by the affect of multiphase flow behaviour on the single-phase shear stress correlations.

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ACKNOWLEDGEMENT

Foremost I would like to thank Professor Morten Christian Melaaen for the valuable guidance and discussions as my supervisor. His knowledge within multiphase flow and his quick response has been very helpful. Special appreciation to my industrial co-supervisor Dr. Geir Elseth at Statoil for his guidance and input to the study.

The building of the horizontal flow loop to study oil-water flow in inclined pipes was supported financially by Norsk Hydro a.s. Special thanks to the technical research support team, who helped building the facility and to Pål Midtbøen for his support on the gamma instrument.

My great appreciation to Prof. Reidar B. Schüller at the University of Life Sciences at Ås for the fruitful discussions regarding multiphase flow behaviour, for his contribution to the instrumental error analysis and for commenting on the thesis.

I would like to extend my profound gratitude to my present colleagues at Statoil for sharing their experience, knowledge and being a source for inspiration. Special thanks to Prof. Ruben Schulkes for the invaluable guidance and support to the modelling part of this study.

Particularly thanks to Atle J. Gyllensten for conducting the computational fluid dynamic simulations and to my former and present managers Pål Eirik Hedne and Bjørn Meland for their support to finish my PhD study at Statoil. I would give my grateful thanks to Dr. Arne Valle and Dr. George William Johnson for helpful discussions.

Sincere gratitude to my former PhD colleagues Sondre Vestøl, Amaranath Sena Kumara, Jon Are Julshamn, Marit Kleven, Anette Mathisen and employees Brit Halvorsen, Wenche Bergland, Thorstein Fåne, Stig Nilsen, Øyvind Johansen and IT services at Telemark University College for contributing to an enjoyable working environment. Specially thanks to Sondre Vestøl for the contribution to the experimental work and to Talleiv Skredtvedt for the first-class technical support on the facility. Especially thanks for the utmost support provided by Øyvind Urkedal, Øystein Gjertsen and Jarle Teigen at the IT services and thanks to my dear beloved friend Gry Sofie Fahrendorff for her comments on my abstract in a busy schedule as a mother of a young child.

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ACKNOWLEDGEMENT

An experimental study of oil-water flow in horizontal and inclined pipes Acknowledgement

ACKNOWLEDGEMENT

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5 Thanks to the summer students at Telemark University College and at Statoil for contributing to both experimental and modelling work. Particular, thanks to Camilla Dyvik for the contribution to the experimental work at Telemark University College and to Marte Kristine Valle and Vibeke Bredvold Karlsen for contributing with the model comparison.

Thanks to the summer students at Telemark University College and at Statoil for contributing to both experimental and modelling work. Particular, thanks to Camilla Dyvik for the contribution to the experimental work at Telemark University College and to Marte Kristine Valle and Vibeke Bredvold Karlsen for contributing with the model comparison.

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NOMENCLATURE

Roman symbols Unit

A Radius of the fluid particle (droplet) (m)

A Area (m²)

Am Atomic mass number (kg/kmol)

API gravity American Petroleum Institute gravity (°API)

C Speed of light in vacuum (m/s)

2 1,c

c Constants in mixture viscosity correlation (-)

CD Particle drag coefficient (-)

cm Speed of light in a substance (m/s)

CH Tunable constant, Eq. (2.20) (-)

CFD Computational fluid dynamics (-)

D Inner pipe diameter (m)

df Fringe distance (m)

Dg Gamma beam diameter (m)

D_H Hydraulic diameter (m)

D_o Outer pipe diameter (m)

D Drop diameter (m)

dmax Maximum droplet diameter (m)

) ( w

d ε Total measurement error in phase fraction (-)

w I

d(ε ) Measurement error due to error in transmitting gamma intensity

(-) )0

( _I

d εw Measurement error due to error in incident/source gamma intensity

(-)

)1

(ε_w _γ

d Measurement error due to error in linear attenuation coefficient for fluid 1.

(-) )2

(εw _γ

(-) w H

d(ε ) Measurement error due to error in liquid length (-)

dmax Maximum droplet size in a turbulent flow (m)

dP Pressure gradient (Pa/m)

dpf Frictional pressure drop (Pa)

dpm Measured pressure drop (Pa)

dps Static pressure drop (Pa)

dpT Total pressure drop (Pa)

D(x) Instrumental accuracy (-)

d_step Uncertainty in the traversing device (-)

Dz0 Uncertainty in zero point (-)

E Gamma energy (keV)

Ec Convergence criteria (-)

F Friction factor (-)

f int Interfacial friction factor (-)

fD Doppler frequency (1/s)

F_D Drag forces on a spherical particle (N)

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NOMENCLATURE

A Area (m²)

2 1,c

D_H Hydraulic diameter (m)

D_o Outer pipe diameter (m)

D Drop diameter (m)

dmax Maximum droplet diameter (m)

) ( w

w I

(-) )0

( _I

d εw Measurement error due to error in incident/source gamma intensity

(-)

)1

(ε_w _γ

(-) )2

(εw_γ

(-) w H

dps Static pressure drop (Pa)

d_step Uncertainty in the traversing device (-)

Ec Convergence criteria (-)

F_D Drag forces on a spherical particle (N)

An experimental study of oil-water flow in horizontal and inclined pipes Nomenclature

NOMENCLATURE

A Area (m²)

2 1,c

d_f Fringe distance (m)

DH Hydraulic diameter (m)

Do Outer pipe diameter (m)

D Drop diameter (m)

d_max Maximum droplet diameter (m)

) ( _w

w I

(-) )0

( w I

d ε Measurement error due to error in incident/source gamma intensity

(-)

)1

(ε_w _γ

(-) )2

(εw _γ

(-) w H

dp_s Static pressure drop (Pa)

dstep Uncertainty in the traversing device (-)

E_c Convergence criteria (-)

FD Drag forces on a spherical particle (N)

NOMENCLATURE

A Area (m²)

2 1,c

d_f Fringe distance (m)

DH Hydraulic diameter (m)

Do Outer pipe diameter (m)

D Drop diameter (m)

d_max Maximum droplet diameter (m)

) ( _w

w I

(-) )0

( w I

d ε Measurement error due to error in incident/source gamma intensity

(-)

)1

(ε_w _γ

(-) )2

(εw_γ

(-) w H

dp_s Static pressure drop (Pa)

dstep Uncertainty in the traversing device (-)

E_c Convergence criteria (-)

FD Drag forces on a spherical particle (N)

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fe Friction factor of finely dispersed emulsion (-)

Fg Gravity force acting on an individual drop (N)

f_ow Interfacial friction factor (-)

FT Net driving force on a drop due to turbulent fluctuation

(N)

g Gravity constant (m/s²)

gN Normal gravity component (m/s²)

gZ Parallel gravity component (m/s²)

h Interfacial height (m)

H Local pipe height between the inner walls (m)

Ho Local pipe height between the outer walls (m)

I Transmitting gamma intensity (1/s)

I0 Incident/source gamma intensity (1/s)

I0’

Initial gamma intensity in air (1/s)

Io Transmitting gamma intensity in oil filled pipe (1/s) I_w Transmitting gamma intensity in water filled pipe (1/s) Ipipe Transmitting gamma intensity in pipe filled with air (1/s)

J Stability criteria parameters (m²/s²)

Js Stability criteria parameters, sheltering effect (m²/s²)

K Concentration function constant (-)

L Pipe length (Axial direction) (m)

lk Kolmogorov microscale

N Number of absorbed gamma ray photons (-)

n0 Initial number of gamma ray photons (-)

n Number of measurements (-)

NA Avogadro number (-)

nr Refraction of light, Snell’s law (-)

ns Sedimentation velocity exponent (-)

NVi Dimensionless viscosity group (Eq. 2.13) (-)

NWe Generelized Weber number (Eq. 2.12) (-)

(NWe)crit Critical Weber number (-)

P Pressure (Pa)

R Inner pipe radius (m)

Ro Outer pipe radius (m)

Re Reynolds number (-)

Recrit Critical Reynolds number (-)

Rep Particle Reynolds number (-)

rms Root mean square (-)

Q Input volumetric flow rate (m³/s)

QCV Quick Closing Valves

S Perimeter of the pipe surface (m²)

SG Specific gravity (-)

Si Width of interface (m²)

Sv Maximum velocity gradient in external flow field (1/s)

STD Standard deviation (-)

Sow Holdup ratio/slip ratio between oil and water (-)

T Time (s)

T int Integral fluid time scale (s)

Tu Turbulence intensity (-)

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f_ow Interfacial friction factor (-)

(N)

I0 Incident/source gamma intensity (1/s)

I0’

Initial gamma intensity in air (1/s)

Io Transmitting gamma intensity in oil filled pipe (1/s) I_w Transmitting gamma intensity in water filled pipe (1/s) Ipipe Transmitting gamma intensity in pipe filled with air (1/s)

Js Stability criteria parameters, sheltering effect (m²/s²)

P Pressure (Pa)

Rep Particle Reynolds number (-)

T Time (s)

fow Interfacial friction factor (-)

(N)

I₀ Incident/source gamma intensity (1/s)

I0’ Initial gamma intensity in air (1/s)

Io Transmitting gamma intensity in oil filled pipe (1/s) Iw Transmitting gamma intensity in water filled pipe (1/s) Ipipe Transmitting gamma intensity in pipe filled with air (1/s)

J_s Stability criteria parameters, sheltering effect (m²/s²)

P Pressure (Pa)

Re_p Particle Reynolds number (-)

T Time (s)

fow Interfacial friction factor (-)

(N)

I₀ Incident/source gamma intensity (1/s)

I0’ Initial gamma intensity in air (1/s)

Io Transmitting gamma intensity in oil filled pipe (1/s) Iw Transmitting gamma intensity in water filled pipe (1/s) Ipipe Transmitting gamma intensity in pipe filled with air (1/s)

J_s Stability criteria parameters, sheltering effect (m²/s²)

P Pressure (Pa)

Re_p Particle Reynolds number (-)

T Time (s)